Hello,
Plaster is actually quite good. Cast ceramic should be better suited.
Slate is not the same as slate. Roof slate, floor slate, etc. is not suitable.
If you want I can tell you an address where you can order good engraved slate in Germany, at a reasonable price. (I think the shipping will be more expensive than the slate). I can also give you an address where you can order the tool for the engraving.
Greetings Gerald